The materials buckling of four BeO moderated U235/aluminium alloy fuelled systems having BeO/U235 ratios of 1465, 2930, 5860, and 8790 have been measured by the exponential method. Relative fission rates of U235, U233, and Pu239 were also measured in the equilibrium spectrum region of the same assemblies. The experiments are described in detail, and the results compared with the predictions of a simple spectrum model (Westcott) pending analysis using more complex calculatlonal models.